1. Field of the Invention
The present invention relates generally to optical communications, and more particularly relates to an apparatus and a method for measuring optical signal-to-noise ratio in optical communications. An optical amplifier is a device to amplify optical signals without photo-electric/electric-photo conversion, and does not depend on the transmission speed or the transmission format of optical signals. A conventional optical transmitter amplifies the signals after photo-electric conversion, and reproduces optical signals with electric-photo conversion. The optical amplifier is substituted for the conventional optical transmitter. In particular, the optical amplifier can simultaneously amplify many optical signals of different wavelengths in a wide band, and the optical amplifier plays an important role in Wavelength Division Multiplexing (WDM) optical communications. In order to increase the transmitting capacity, several channels of optical signals are multiply divided with different wavelengths in WDM
2. Description of the Related Art
The optical amplifier produces not only the amplified optical signals but also the noise of wide wavelength band. Even though optical filters are used to remove the noise included in the output beam of the optical amplifier, the noise of same wavelengths of the signals can not be eliminated. Optical signal-to-noise ratio, the power of the optical signals divided by the power of the corresponding noise, is used as a standard for a transmitting quality of optical communication network. And it is necessary to monitor the optical signal-to-noise ratio at optical lines or at optical nodes for optical communication network management.
FIG. 1 is a power spectrum of the output beam when the optical amplifier amplifies the multiplexed signals without optical filters in WDM optical communications. The spectrum is taken from a conventional spectrum analyzer. As mentioned above, the optical signal-to-noise ratio is the value of the power of the optical signals divided by the power of the corresponding noise. However, the noise can not be measured directly since the noise are detected with the optical signals as shown in FIG. 1.
FIG. 2 is a graph to explain how to measure the optical signal-to-noise ratio of output signals shown in FIG. 1. In order to measure the ratio, first take a power spectrum of the multiplexed signals amplified with the optical amplifier. From the obtained spectrum, find A, the peak power of the Optical Signal 1, and measure a, b, the neighboring noise power. Calculate (a+b)/2, the average noise-power, and assume it as the noise power at the wavelength of the Optical Signal 1. Then, the optical signal-to-noise ratio of the Optical Signal 1 is obtained using EQUATION 1.                               Optical  Signal-to-noise  ratio
    of  the  Optical  Signal  1                =                              A            -                                          (                                  a                  +                  b                                )                            /              2                                                          (                              a                +                b                            )                        /            2                                              [                  EQUATION          ⁢                      xe2x80x83                    ⁢          1                ]            
Similarly, the ratio of the Optical Signal 2, and Signal 3 can be obtained.
However, in some cases, it is impossible to measure the optical signal-to-noise ratio with the above method in FIG. 2. FIG. 3 explains these cases, and shows another power spectrum of the output beam when optical filters are used to remove the noise in WDM optical communications. According to the method in FIG. 2, the power (a, b) of noise nearby the peak wavelength should be known in order to measure the optical signal-to-noise ratio. In cases of FIG. 3, the method in FIG. 2 can not be used since the noise is not easily distinguishable from the optical signals.
FIG. 4 is a block diagram to solve the problems, and shows a device to measure the optical signal-to-noise ratio using a polarization controller and a linear polarizer (LP). The optical signal-to-noise ratio in FIG. 3 can be measured with the instruments in FIG. 4. The device (400) for measuring the optical signal-to-noise ratio shown in FIG. 4 is published in ""98 European Conference on Optical Communication, p. 549-550, 1998"" with the title of xe2x80x9cOptical Signal-to-Noise Ratio Measurement in WDM Networks Using Polarization Extinctionxe2x80x9d by M. Rasztovits-Wiech, M. Danner, and W. R. Leeb.
In optical communications, laser diodes are generally used as a light source. The polarization state of the output beam from a laser diode is 100% linearly polarized, and the optical signals are still 100% polarized even though the polarization state is changed as the signals traveling the optical fiber. On the other hand, the noise from an optical amplifier is 100% unpolarized since the noise consists of the randomly occurred lights of all polarization states.
Therefore, the power of the interested noise can be measured when the amplified optical signals are eliminated using a polarization controller (401) and a LP (402). The polarization controller controls the polarization of the input beam, and the LP passes only the component of the light coincide with the polarization axis. The 100% polarized optical signals can be completely eliminated; the polarization controller (401) can control the polarization state of the optical signals even after the signals traveled the optical fiber, and the controller changes the polarization of the optical signals orthogonal to the polarization axis of the LP (402). However, the power of the noise passing through a LP (402) always reduces to the half since the noise consists of the lights of all polarization states.
Referring FIG. 4, the output beam (shown in FIG. 1) of the optical amplifier is inputted into the polarization controller (401). Adjust the polarization controller to maximize the power of the optical signals passing through the LP (402) and the variable optical band-pass filter, VOBPF (403) at the photo detector (404), and measure the maximum value. Then, readjust the polarization controller to minimize the power at the photo detector (404), and measure the minimum value. Repeat the process for the full spectrum range by changing the passing wavelength of the VOBPF (403).
FIG. 5(a) is a spectrum of the output beam in FIG. 1 when the power at the photo detector (404) in FIG. 4 is maximized, and FIG. 5(b) is another spectrum when the power is minimized. In FIG. 5(a), the peak power is the sum of the power, D, of the optical signals and the half, d, of the noise power, while the power in FIG. 5(b) is the half of the original noise. Then, the optical signal-to-noise ratio of the Optical Signal 1 is obtained using EQUATION 2.                               Optical  Signal-to-noise  ratio
    of  the  Optical  Signal  1                =                              D            -            d                                2            xc3x97            d                                              [                  EQUATION          ⁢                      xe2x80x83                    ⁢          2                ]            
Similarly, the ratio of the Optical Signal 2, and Signal 3 can be obtained.
FIG. 6(a) is a spectrum of the output beam in FIG. 3 when the power at the photo detector (404) in FIG. 4 is maximized, and FIG. 6(b) is another spectrum when the power is minimized. In FIG. 6(a), the peak power is the sum of the power, E, of the optical signals and the half, e, of the noise power, while the power in FIG. 6(b) is the half of the original noise. Therefore, the optical signal-to-noise ratio of the Optical Signal 1 is obtained using EQUATION 3.                               Optical  Signal-to-noise  ratio
    of  the  Optical  Signal  1                =                              E            -            e                                2            xc3x97            e                                              [                  EQUATION          ⁢                      xe2x80x83                    ⁢          3                ]            
Similarly, the ratio of the Optical Signal 2, and Signal 3 can be obtained.
However, the preceding method shown in FIG. 4 needs to adjusts the polarization controller (401) to find the maximum and the minimum of the optical power at the photo detector (404) for each given wavelength. And the method has two major problems; (1) long operation time to measure the optical signal-to-noise ratio, and (2) a complicated active control circuit to handle the polarization controller (401).
It is an object of the present invention to provide an apparatus and a method for measuring the optical signal-to-noise ratio using Stokes parameters in optical communications. The present invention can solve the above mentioned problems.
In accordance with an aspect of the present invention there are provided an apparatus and a method for measuring the optical signal-to-noise ratio in optical communications. The apparatus according to the invention comprises (1) a VOBPF passing the amplified output beam when the beam wavelength is same as the passing wavelength of the VOBPF; (2) a 1xc3x974 beam distributor distributing the passing beam of the VOBPF into four streams; (3) a measuring means of Stokes parameters S0, S1, S2, S3 from the four distributed beams; (4) a calculating means of the optical signal power finding the power of the polarized component of the amplified output beam from the Stokes parameters S1, S2, S3; (5) a calculating means of the noise power finding the power of the noise included in the amplified output beam from Stokes parameter S0 and the optical signal power; and (6) a dividing means calculating the       Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Optical    ⁢          xe2x80x83        ⁢    Signal        Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Noise  
of the passing wavelength from the optical signal power and the noise power. The apparatus according to the invention measures the       Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Optical    ⁢          xe2x80x83        ⁢    Signal        Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Noise  
for the whole spectrum range by changing the passing wavelength of the VOBPF, and eventually find the optical signal-to-noise ratio of an optical signal by searching the peak from the measured       Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Optical    ⁢          xe2x80x83        ⁢    Signal        Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Noise  
graph.
In a preferred apparatus, the measuring means of Stokes parameters S0, S1, S2, S3 further comprises (1) 0xc2x0 linear polarization means detecting the power of 0xc2x0 linear polarization component (PX) from the first distributed beam; (2) 90xc2x0 linear polarization means detecting the power of 90xc2x0 linear polarization component (PY) from the second distributed beam; (3) 45xc2x0 linear polarization means detecting the power of 45xc2x0 linear polarization component (P45) from the third distributed beam; (4) circular polarization means detecting the power of right-hand circular polarization component (PRCP) from the forth distributed beam; and (5) a Stokes parameter calculating means finding Stokes parameters S0, S1, S2, S3 from the measured power PX, PY, P45, PRCP
In a more preferred apparatus, the circular polarization means further comprises (1) a xcex/4 retarder making the phase difference between the two orthogonal beams with 0xc2x0 and 90xc2x0 linear polarization become xcex/4 by retarding the phase of the input beam, and (2) a 45xc2x0 LP passing only the 45xc2x0 linear polarization component after the xcex/4 retarder.
In another more preferred apparatus, the Stokes parameter calculating means further comprises (1) a first adder finding Stokes parameter S0 by adding the power PX and PY; (2) a second adder finding Stokes parameter S1 by subtracting the power PY from PX; (3) a first multiplier multiplying the power P45 by 2; (4) a third adder finding Stokes parameter S2 by subtracting Stokes parameter S0 from the output of the first multiplier, 2P45; (5) a second multiplier multiplying the power PRCP by 2; and (6) a fourth adder finding Stokes parameter S3 by subtracting Stokes parameter S0 from the output of the second multiplier, 2PRCP.
In another preferred apparatus, the VOBPF can be embodied with a Fabry-Perot variable filter, an integrated optic device with lattices, or a multi-layered thin film.
A method for measuring the optical signal-to-noise ratio in optical communications according to the invention comprises following 8 steps. (1) A first step where the amplified output beam passes through the VOBPF with the starting wavelength; (2) a second step where the passing beam of the first step is distributed into four streams; (3) a third step where obtain Stokes parameters S0, S1, S2, S3 from the four distributed beams; (4) a fourth step where find the optical signal power by calculating the power of the polarized component of the amplified output beam from the Stokes parameters S1, S2, S3; (5) a fifth step where find the power of the noise included in the amplified output beam from Stokes parameter S0 and the optical signal power; (6) a sixth step where calculate       Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Optical    ⁢          xe2x80x83        ⁢    Signal        Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Noise  
of the passing wavelength from the optical signal power and the noise power; (7) a seventh step where measure       Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Optical    ⁢          xe2x80x83        ⁢    Signal        Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Noise  
for the whole spectrum range by repeating the process from the second step to the sixth step for the each passing wavelength of the VOBPF; and (8) a eighth step where find the optical signal-to-noise ratio of an optical signal by searching the peak from the measured       Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Optical    ⁢          xe2x80x83        ⁢    Signal        Power    ⁢          xe2x80x83        ⁢    of    ⁢          xe2x80x83        ⁢    Noise  
graph.
In a preferred method, the third step further comprises (1) a first sub-step where detect the power PX, PY, P45, PRCP; and (2) a second sub-step where calculate Stokes parameters S0, S1, S2, S3 from the measured power PX, PY, P45, PRCP.
In a more preferred method, the second sub-step further comprises following four steps. (1) Finding Stokes parameter S0 by adding the power PX and PY; (2) finding Stokes parameter S1 by subtracting the power PY from PX; (3) finding Stokes parameter S2 by subtracting Stokes parameter S0 from 2P45; and (4) finding Stokes parameter S3 by subtracting Stokes parameter S0 from 2PRCP.